Explanation of error Unresolved URI used for object identification

XMI allows reference to objects defined in external files. Reference to these
objects is provided by URIs. This method of reference is typically
used to refer to the standard UML Stereotypes, objects in a Profile,
or in the case that the test data is a meta-model, UML itself.
Standard URIs are associated with the aforementioned specifications. The URI used
in the case where this error was signaled was not a standard URI known to
the Validator. URLs for models known to the validator can be found
here.

You can use the 'Load Metamodels' menu
item on the left to load models and specify the URI by which they should be dereferenced.